1. Field of the Invention
This invention relates to an active filter circuit combining an operational amplifier, a capacitor and a resistor, and more particularly to the same fabricated on a single semiconductor chip as an integrated circuit (IC).
2. Description of the Prior Art
Active filter circuits are widely used for a variety of purposes of removing the quantizing noise associated with A/D conversion and shaping signal waveforms. The capacitance value of the capacitor and the resistance value of the resistor depend on the resistivity and dielectric constant of materials and thus vary with geometry and dimensions thereof. As well known in the art, these values are inevitably deviated in the IC from designed values by about 20 to about 30% due to the occasional state of the material and ununiform temperature in the fabrication process and by precision for processing into a desired geometry. Consequently, it is usual for the cut-off frequency and the pass-band width of the active filter circuit to deviate much from the designed values.
In order to correct deviation of the above-mentioned cut-off frequency and pass-band width, there are fabricated on such an IC chip a circuit consisting of a capacitor and a plurality of resistors interconnected in series or parallel thereto. Some of the resistors are provided with respective shunt fuses to be selectively blown for adjusting the summed resistance of the circuit on the measured result after completion of the semiconductor chip. This adjustment is usually carried out to keep at a certain value the time constant, that is, the product of the capacitance and resistance of the RC circuit, and therefore the minimum value of the summed resistance when no fuses are blown is set to correspond to the largest allowable deviation of the capacitance. Another method is used which is a laser-trimming technique of previously fabricating a square-film resistor connected in series to the capacitor on the chip and cutting a slit of a suitable length in the resistor film by means of laser beam so as to set its resistance to a desired value.
In addition to the methods of fuse-blowing and laser trimming, some methods of self-correcting the values of capacitor and resistor have been proposed. For example, an article entitled "High frequency CMOS continuous time filters" IEEE Journal of Solid-State Circuits, vol. SC-19, December 1984, pp. 939-948 describes a voltage-controlled oscillator (VCO) and a phase comparator, these constituting a phase locked loop (PLL), and an active filter, all built on the same chip. The phase comparator compares in phase between the output of the VCO and an external reference frequency or high precision clock signal and generating control voltages. The voltage-controlled oscillator includes a CR integrator circuit controlled by the cotrol voltages, in which a voltage-controlled current source having a mutual conductance is contained. The active filter contains CR integrators of the same type as the CR integrators in the VCO. The (oscillation) frequency of VCO is synchronized with the above-mentioned clock signals, whereby the time constant of the above-said integrator circuits is corrected. The above-said common control voltages are applied not only the VCO but also the integrators which are components of the filter, and therefore correction of these integrators on the same chip is carried out simultaneously as the integrator circuit of VCO.
Another article entitled "Gyrator video filter IC with automatic tuning" IEEE Journal of Solid-State Circuits, Vol. SC-15, November 1980, pp. 963-968 describes a filter for TV video signal frequency band which has a self-corrector circuit with a variable bipolar-transistor gyrator as a phase shifter.
In addition, Japanese Published Patent No. Hei.4-73886 specification describes a method of adjusting the time constant of a built-in active filter of the integrated circuit by using a time constant-variable current source.
On the active filter circuits in the prior art, those processed by the above-mentioned fuse blowing and laser trimming need equipment for checking capacitances and resistances, fuse blowing, and laser processing, and the correspondingly-added processing steps, with correspondingly-increased manufacture costs. Besides, fuse blowing and laser beam irradiation provide stress on the semiconductor substrate and the devices/interconnections on its surface, resulting in decreased reliability of the semiconductor IC itself including such a filter circuit as mentioned above.
In addition the above-mentioned self-correcting circuits need current source or variable gyrator and many elements for the associated peripheral circuits, these not only taking up large surface on the substrate surface but also imposing limitation on the operational frequency range.